Magnetron sputtering is commonly used to coat substrates with a thin film using physical vapor deposition, PVD. Sputter coating technology is used for producing thin films in, for example, the semiconductor industry, the optical industry, the data storage industry and also for producing functional films or coatings for other applications. Magnetron sputtering sources, more precisely magnetron cathode sputtering sources, are widely used because they provide high deposition rates. A semiconductor wafer used in the semiconductor manufacturing industry or other work piece used in other industries, is positioned in a processing chamber of a magnetron sputtering system, a sputtering target is affixed within the system and electrical and magnetic fields are generated to cause the target material to erode and the liberated target material to be directed to the semiconductor wafer or other work piece.
More particularly, in sputtering cathodes, the coating material provided in the form of a target is eroded by energetic ions from a plasma discharge and the material liberated by the ions forms a thin film coating on the substrate via physical vapor deposition, PVD. The plasma discharge is generally maintained in an evacuated process chamber, i.e. a vacuum chamber, under controlled flow of a working gas with an electric potential and discharge current applied by a power supply between the target cathode and an anode.
In the standard case of electrically conductive target materials, the target is supplied with a continuous or pulsating negative voltage, such that a plasma forms above the target surface. By means of an electrical field formed between the plasma and target surface, positively charged ions from the plasma are accelerated toward and onto the negatively biased target surface, i.e. the cathode, bombarding the target surface and causing erosion of the target by freeing material from the target and resulting in material being sputtered away from the target surface. The liberated material from the eroding target is directed from the cathode to a semiconductor substrate or other workpiece positioned on a support maintained at or near the anode potential thereby coating the semiconductor substrate or workpiece. For electrically non-conductive target materials, the plasma discharge is maintained using a high frequency supply leading to ion bombardment of the target pulsating with the high-frequency, thus resulting in a sputter erosion process of the target.
The material liberated from the target deposits on the substrate in approximately the same composition as present in the target.
In a magnetron sputtering system, the plasma density above the target is strongly increased by means of magnetic fields. Ions in the high plasma density region produced by the magnetic field, become highly energized. The magnetic fields are produced by an arrangement of magnets arranged in close proximity and in fixed position with respect to the target. The magnet arrangement is typically disposed on the side of the target opposite the target sputtering surface and outside the process chamber. When the magnetron sputtering system is in operation, the magnetic field lines produced by the magnet arrangement are in fixed location with respect to the magnets and with respect to the target since the target mount is in fixed position with respect to the magnet arrangement. The targets may take on various geometric shapes such as oval, circular, or rectangular. The substrate or substrates being coated may be in movement or stationary during the coating process.
In conventional magnetron sputtering systems, however, the target will have an uneven erosion profile. Regardless of its shape, the target erodes more preferentially at specific locations with respect to the fixed magnetic fields that are produced by the fixed magnet arrangement. When a magnet arrangement is located behind the target, the resulting magnetic field forms a closed-loop annular path acting as an electron trap that reshapes the trajectories of the electrons. The magnetic field lines created by the magnet arrangement determine the erosion pattern of the target because the electrons which create the plasma discharge are energized by the magnetic field and accelerated toward the target at a 90 degree angle relative to the magnetic field lines. Once the erosion profile has been established on the target surface, it remains constant throughout the life of the target.
The uneven erosion profile of the target can result in poor uniformity of the deposited film, i.e. coating.
The target actually consists of the target material that is to be sputtered as a coating, disposed on a backing plate which may be a ferromagnetic or other metallic material. When the spatially preferential erosion of particular locations on the target causes the target material to be completely consumed in these locations and the backing material exposed, problems arise. In particular, the material of the backing plate of the target becomes attacked by the plasma ions and can become eroded and undesirably form part of the coating. This contaminates the coating and the system. As such, the target must be changed prior to the target material being completely consumed at any one location. This results in the frequent disposal of targets that still include very thick portions of useable target material in the non-eroded portions, and is cost ineffective.
It is clear that it would be desirable to avoid this situation and to increase full target utilization, and also to ensure uniform coating on the substrates.